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Identifying biological traits that promote evolutionary success is fundamental for understanding biodiversity dynamics and for assessing the evolutionary response of organisms to global change. We tested the hypothesis that image-forming eyes have contributed to the diversification of taxa in the geological past. Using fossil occurrences in the Paleobiology Database, we analyzed the diversity and evolutionary rates of more than 17,000 Phanerozoic genera of marine invertebrates living on or above the shallow-water seafloor according to their visual capabilities. Analysis of the complete data set shows a peak in the proportional diversity of sighted genera early in the Phanerozoic, and their continuance at a relatively low and stable level after the Ordovician. As an explanation of this pattern we suggest that selection pressure to develop eyes rose in the Cambrian, and that behavioral constraints had a balancing effect thereafter. In contrast to the pooled data, a clade-level study of those subgroups that contain both sighted and blind genera revealed that—in trilobites, all epifaunal bivalves, pectinoid bivalves, gastropods, and echinoderms—sighted genera diversified more strongly than blind genera. This difference is controlled by significantly raised extinction rates of blind genera. These more finely resolved patterns support the hypothesis that good vision is a key trait that promoted preferential diversification.
Analysis of a global elevation database to measure changes in shallow-marine habitat area as a function of sea level reveals an unexpectedly complicated relationship. In contrast to prevailing views, sea level rise does not consistently generate an increase in shelf area, nor does sea level fall consistently reduce shelf area. Different depth-defined habitats on the same margin will experience different changes in area for the same sea level change, and different margins will likewise experience different changes in area for the same sea level change. Simple forward models incorporating a species-area relationship suggest that the diversity response to sea level change will be largely idiosyncratic. The change in habitat area is highly dependent on the starting position of sea level, the amount and direction of sea level change, and the habitat and region in question. Such an idiosyncratic relationship between diversity and sea level reconciles the widespread evidence from the fossil record for a link between diversity and sea level change with the lack of quantitative support for such a relationship throughout the Phanerozoic.
The late Mesozoic through early Cenozoic is an interval of significant biologic turnover and ecologic reorganization within marine assemblages, but the timing and causes of these changes remain poorly understood. Here, we quantify the pattern and timing of shifts in the diversity (richness and evenness) and ecology of local (i.e., sample level) mollusk-dominated assemblages during this critical interval using field-collected and published data sets from the U.S. Gulf Coastal Plain. We test whether the biologic and ecologic patterns observed primarily at the global level during this time are also expressed at the local level, and whether the end-Cretaceous (K/Pg) mass extinction and recovery moderated these trends. To explore whether environment had any effect on these patterns, we examine data from shallow subtidal and offshore settings.
Assemblages from both settings recovered to pre-extinction diversity levels rapidly, in less than 7 million years. Following initial recovery, diversity remained unchanged in both settings. The trajectory of ecological restructuring was distinct for each setting in the wake of the K/Pg extinction. In offshore assemblages, the abundance and number of predatory carnivorous taxa dramatically increased, and surficial sessile suspension feeders were replaced by more active suspension feeders. In contrast, shallow subtidal assemblages did not experience ecological reorganization following the K/Pg extinction. The distinct ecological patterns displayed in each environment follow onshore-offshore patterns of innovation, whereby evolutionary novelties first appear in onshore settings relative to offshore habitats. Increased predation pressure may explain the significant ecological restructuring of offshore assemblages, whereby the explosive radiation of predators drove changes in their prey. Habitat-specific ecological restructuring, and its occurrence solely during the recovery interval, implies that disturbance and incumbency were also key in mediating these ecological changes.
High-resolution palynological data sets from shallow marine Triassic-Jurassic (Tr/J) boundary beds of two principal sections in Europe (Hochalplgraben in Austria and St. Audrie's Bay in the United Kingdom) were analyzed to reconstruct changes in vegetation, biodiversity, and climate. In Hochalplgraben, a hardwood gymnosperm forest with conifers and seed ferns is replaced by vegetation with dominant ferns, club mosses and liverworts, which concurs with an increased diversification of spore types during the latest Rhaetian. Multivariate statistical analysis reveals a trend to warmer and wetter conditions across the Tr/J boundary in Hochalplgraben. The vegetation changes in St. Audrie's Bay are markedly different. Here, a mixed gymnosperm forest is replaced by monotonous vegetation consisting mainly of Cheirolepidiaceae (80–100%). This change is caused by a transition to a warmer and more arid climate. The observed diversity decrease in St. Audrie's Bay affirms this interpretation. Although both sections show major vegetation changes, neither of them demonstrates a distinctive floral mass extinction. A compilation of Tr/J boundary sections across the world demonstrates the presence of Cheirolepidiaceae-dominated forests in the Pangaean interior and increases in abundance of spore-producing plants adjacent to the Tethys Ocean. We propose that the non-uniform vegetation changes reflected in the Tr/J palynological records are the result of environmental changes caused by Central Atlantic Magmatic Province volcanism. The increase in greenhouse gases caused a warmer climate and an enhanced thermal contrast between the continent and the seas. Consequently, the monsoon system got stronger and induced a drier continental interior and more intensive rainfall near the margins of the Tethys Ocean.
A key question in studies of mass extinctions is whether the extinction was a sudden or gradual event. This question may be addressed by examining the locations of fossil occurrences in a stratigraphic section. However, the fossil record can be consistent with both sudden and gradual extinctions. Rather than being limited to rejecting or not rejecting a particular scenario, ideally we should estimate the range of extinction scenarios that is consistent with the fossil record. In other words, rather than testing the simplified distinction of “sudden versus gradual,” we should be asking, “How gradual?”
In this paper we answer the question “How gradual could the extinction have been?” by developing a confidence interval for the duration of a mass extinction. We define the duration of the extinction as the time or stratigraphic thickness between the first and last taxon to go extinct, which we denote by Δ. For example, we would like to be able to say with 90% confidence that the extinction took place over a duration of 0.3 to 1.1 million years, or 24 to 57 meters of stratigraphic thickness. Our method does not deny the possibility of a truly simultaneous extinction; rather, in this framework, a simultaneous extinction is one whose value of Δ is equal to zero years or meters.
We present an algorithm to derive such estimates and show that it produces valid confidence intervals. We illustrate its use with data from Late Permian ostracodes from Meishan, China, and Late Cretaceous ammonites from Seymour Island, Antarctica.
A basic hypothesis in extinction theory predicts that more abundant taxa have an evolutionary advantage over less abundant taxa, which should manifest as increased survivorship during major extinction events and longer fossil-record durations. Despite this, various paleontologic studies have found conflicting patterns, indicating a more complex relationship between abundance and extinction in the geologic past. This study tests the relationship between abundance and extinction among brachiopod genera within seven third-order depositional sequences spanning the Late Ordovician to Early Silurian (Katian–Aeronian) of the Cincinnati Arch.
Contrary to predictions, abundance is not positively correlated with duration in this study. Abundance and duration range from strongly negatively correlated to uncorrelated depending on the spatial scale of analysis and the geologic intervals included, but correlations never indicate that abundance is an evolutionary advantage. In contrast, abundance was an advantageous trait prior to the Ordovician/Silurian extinction, and brachiopods with higher abundances were more likely to survive the event than less abundant brachiopods. While this result is in keeping with common models of extinction, it has not been observed previously at a mass extinction boundary. This may be further evidence that the Ordovician/Silurian extinction was not accompanied by a shift in the macroevolutionary selectivity regime.
The fossil record provides an important source of data on adaptive radiations, and indeed some of the earliest theoretical insights on the nature of these radiations were made by paleontologists. Here we focus on the diverse Devonian Metacryphaeus group calmoniid trilobites, known from the Malvinokaffric Realm, which have been considered a classic example of an adaptive radiation preserved in the fossil record. We use a geometric morphometric analysis in conjunction with phylogenetic and biogeographic patterns and data on speciation rates. Using ancestral character state reconstruction during speciation events, we quantify patterns of morphological change in order to assess the role ecological and geographical factors may have played in mediating this radiation. We found no significant differences between the amount of morphological change that occurred during speciation events when ancestors and descendants were in the same area as opposed to when they occupied different areas. Further, the magnitude of morphological divergence did not change through time or with cladogenetic rank. These patterns, in conjunction with the fact that the radiation occurs in a geographically heterogeneous region subjected to repeated episodes of sea-level rise and fall, suggest that at the macroevolutionary scale this radiation may have been motivated more by phenomena that facilitated geographic isolation than by competition.
A developmental model, based upon murine rodents, has been proposed by Kavanagh et al. (2007) to explain lower molar proportions in mammals. We produce a clade-wide macroevolutionary test of the model using the dental evolutionary trends in a unique radiation of extinct mammals endemic to South America (“Meridiungulata”) that comprise a diverse array of molar morphologies. All of the South American ungulate groups examined follow the inhibitory cascade model with the exception of two groups: Interatheriidae (Notoungulata) and Astrapotheria. For most taxa studied, ratios between lower molar areas are greater than 1.0, indicating a weak inhibition by m1 on the subsequent molars in the tooth row, and a trend to greater absolute size of the posterior molars. Comparisons of mean ratios between clades indicate that a significant phylogenetic signal can be detected, particularly between the two groups within Notoungulata— Typotheria and Toxodontia. Body mass estimates were found to be significantly correlated with both m3/m1 and m2/m1 ratios, suggesting that the larger body size achieved the weaker inhibition between the lower molars. Molar ratio patterns are examined and discussed in relation to the independent and numerous acquisitions of hypsodonty that are characteristic of dental evolution in “Meridiungulata.”
We studied the morphological diversity of gastropod shell forms from the viewpoint of theoretical morphology, emphasizing the relationships of shell form to postural stability and the available space for soft body, which we assessed in terms of the moment of force and soft-tissue ratio calculations, respectively. The results of computer simulations suggest a functional trade-off between postural stability and available space for soft body: a compact shell possessing a low spire and small umbilicus exhibits high postural stability, whereas a less overlapped shell form with a high spire and large umbilicus makes available space for soft body. A functional morphospace analysis using theoretical models reveals that outward and downward inclination of the aperture moderates the functional trade-off between these parameter values and permits compatibility between stable posture and efficient shell construction. The hypothetical optimum that realizes this compatibility is consistent with the observed range of forms estimated from 359 extant gastropod species. The biometric results also suggest that land snails are more highly constrained than marine species in achieving a balance between postural stability and available space for soft body.
The clade Archosauria contains two very different sister groups in terms of diversity (number of species) and disparity (phenotypic variation): Crurotarsi (taxa more closely related to crocodiles than to birds) and Ornithodira (pterosaurs and dinosaurs including birds). The extant species of Crurotarsi may constitute a biased sample of past biodiversity regarding growth patterns and metabolic rates. Bone histological characters can be conserved over hundreds of millions of years in the fossil record and potentially contain information about individual age at death, age at sexual maturity, bone growth rates, and basal metabolic rates of extinct vertebrates. Using a sample of extant amniotes, we have constructed a paleobiological model to estimate bone growth rate from bone histological traits. Cross-validation tests show that this model is reliable. We then used it to estimate bone growth rates in a sample of extinct archosaurs including Crurotarsi and Ornithodira. After testing for phylogenetic signal, optimization of femoral growth rates through squared change parsimony onto a time-calibrated tree of amniotes shows two divergent evolutionary trends: whereas bone growth rates increase from the last common ancestor of Ornithodira to extant birds, they decrease from the last common ancestor of Crurotarsi to extant crocodiles. However, we conclude, on the basis of recent evidence for unidirectional airflow in the lungs of alligators, that crocodiles may have retained the capacity of growing at high rates.